Study of Scattering vs. Transparency for Corneal Graft Optimization

Abstract : Purpose: Femtosecond laser corneal surgery is currently limited in depth for transfixing cuts in oedematous cornea. We perform spectral characterization measurements on corneal tissue over a wide spectral range in order to develop optimized laser sources and to gain new insight in tissue optics. We propose a new and simple optical device to evaluate corneal graft quality. Methods: Human corneas have been exposed to a spatially coherent white light system and used in confocal geometry to perform the ballistic transmission spectrum through the cornea, furthermore the total transmission spectrum has been obtained using an integrating sphere. In parallel, monochromatic light emitting diodes (LED) have been used for imaging a star target through human corneas to obtain its modulation transfer function (MFT) after a numerical analysis. Results: Ballistic and total transmission spectra of the corneas have been obtained, and from those the scattering cross section has been evaluated. Human corneas present a similar absorption spectrum as water. The scattering cross section is significantly lowered at longer wavelengths. The absorption losses which are present in this spectral range are compensated by reduced scattering. Therefore it is possible to perform efficient transfixing cuts in the depth of the cornea using mid-infrared wavelength (around 1.6 µm). The simplified star target system allows an efficient and objective evaluation of the corneal quality of individual donor grafts. Conclusions: Our findings suggest that future improved systems for keratoplasty may obtain optimized performance by using laser sources emitting at longer infrared wavelengths.The optical quality of individual donor corneas may be evaluated using our simplified, standardized optical measurement device.